Topband: Common mode choking of beverages - which side of grounded shack entry panel?

[Jim Brown]

On 10/3/2020 11:24 PM, donovanf at erols.com wrote:
> Double shielded coaxial cable or hard line coax with properly installed
> high quality coaxial connectors greatly minimize the penetration
> of common mode signals into the coaxial cable and the equipment to
> which it is connected.

GM Frank,

Quite true, BUT -- while the CATV RG6 that is widely sold and used for 
RX antennas is advertised as double-shielded, that shielding is thin Al 
braid and foil, which has a relatively high RF resistance. This makes it 
a relatively poor shield compared to hard line and the high quality 
double-copper-braid coaxes like 8214 and RG400.

The relevant spec (which is rarely published except for some high-spec 
cables used in 2-way and cell sites) is the Transfer Impedance, which is 
the ratio of the voltage induced inside the cable to the common mode 
current on the shield. Ott observes that the lower limit is the RF 
resistance of the shield, which is the DC resistance with the multiplier 
for skin effect applied. In addition to that resistance, transfer 
impedance can be further degraded by the construction (materials, 
uniformity, density) of the shield.

So I agree with your recommendation for lines run with hard line or 
double-shielded coaxes like 8214 or RG400, but for that CATV coax, a 
second choke somewhere along the line on the outside of the entry panel 
can reduce noise and crosstalk from transmitters on site, especially on 
very long runs.

As to chokes inside the station at the rig -- virtually every rig I 
visually inspected the last time I was in Dayton had Pin One Problems at 
secondary inputs and outputs. The MAIN TX/RX was properly bonded to the 
chassis at the point of entry, but most other I/O was not. If the 
connector shell for the RX antenna was not bonded to chassis at the 
point of entry, the equipment would be quite susceptible to common mode 
current on that cable shield, and a choke could prevent crosstalk and 
noise at that interface. This can be a small problem in small stations, 
but a much larger one in large stations like yours.

73, Jim K9YC